System for Purifying the Exhaust Gases Emitted by Internal Combustion Engines

ABSTRACT

A system for purifying the exhaust gases emitted by an internal combustion engine ( 2 ), comprising a series-connected catalytic silencer ( 3 ) and antiparticulate filter ( 4 ) which are traversed in succession by an exhaust gas flow leaving the engine ( 2 ), said antiparticulate filter ( 4 ) being located upstream of the catalytic silencer ( 3 ) such as to be traversed by the hot exhaust gases from the engine ( 2 ) before they cool substantially.

TECHNICAL FIELD

The present invention relates to a system for purifying the exhaust gases emitted by automotive internal combustion engines, and in particular by diesel engines.

PRIOR ART

The pollutants contained in diesel engine exhaust gases are known to consist mainly of unburnt hydrocarbons (HC), nitrogen oxides (NOx) and solid carbon particles known commonly as particulate.

To reduce unburnt hydrocarbons and nitrogen oxides, diesel engine purification systems generally comprise a catalytic silencer, which is traversed by the engine exhaust gases and contains catalysts which activate the chemical reactions necessary for reducing the nitrogen oxides and for oxidizing the unburnt hydrocarbons, prior to their release to atmosphere.

In detail, the catalytic silencer comprises a series of active elements which are traversed in succession by the exhaust gases, including an oxidizing precatalyst, a catalyst and a silencer.

To reduce the particulate, purification systems comprise a suitable antiparticulate filter located within the catalytic silencer between the oxidizing precatalyst and the catalyst, it consisting of porous material which retains the particulate and allows the aeriform phase to pass. The most widespread antiparticulate filters are currently of ceramic material, typically silicon carbide, however metal filters, for example of sintered metal, are also known. During use, antiparticulate filters clog generally very rapidly, and are therefore periodically subjected to a regeneration step, in which they are raised to high temperature to enable the trapped carbon particles to burn off and free their filtering walls.

The normal operating temperature of currently used antiparticulate filters is about 300° C., insufficient to ensure particulate combustion, which takes place on raising the operating temperature beyond 600° C.

Said regeneration is hence achieved by introducing into the antiparticulate filters a suitable mixture of oxygen and easily flammable gases, which on contact with the filtering walls, possibly covered with a catalyst, ignite spontaneously.

In particular, said regeneration mixture can be introduced either directly by suitable injectors or indirectly by suitably regulating the air/fuel ratio of the engine feed.

A drawback of known antiparticulate filters is that many regions of their filtering walls do not reach the temperature required for igniting the regeneration mixture. For this reason, combustion takes place only at the hottest points of these filtering walls, which are unable to be completely freed of the accumulated particulate, with consequent loss of system efficiency.

This phenomenon is due substantially to two contributory causes. The first is that during normal operation the antiparticulate filters are heated by the exhaust gases in a generally disuniform manner, with temperature differences which are greater the lower the thermal conductivity of their construction material is.

The second is that during exhaust gas passage, the carbon particles contained in the exhaust gases mutually agglomerate and accumulate disuniformly on the on the filtering walls of the antiparticulate filter. This fact means that within the most heavily clogged regions, not all the accumulated particulate can be effectively burnt off, neither do the conditions for regeneration mixture combustion exist.

DISCLOSURE OF THE INVENTION

The object of the present invention is to overcome the said drawback within the framework of a simple, rational and low-cost solution.

This object is attained by a system for purifying the exhaust gases emitted by an internal combustion engine, comprising a series-connected catalytic silencer and antiparticulate filter, which are traversed in succession by an exhaust gas flow leaving the engine.

According to the invention, the antiparticulate filter is located upstream of the catalytic silencer, as close as possible to the engine, such as to be traversed by the exhaust gases before they cool substantially.

By virtue of this arrangement, the antiparticulate filter is heated to an average operating temperature much higher than in the known art, this enabling all regions of the relative filtering walls to reach the minimum ignition temperature of the regeneration mixture. In this manner, the regeneration process involves the entire antiparticulate filter, the useful life of which is hence effectively increased.

Preferably, the antiparticulate filter is positioned so close to the engine as to reach an average operating temperature exceeding 600° C., so that combustion of the accumulated particulate can take place continuously during normal system operation, without the need to introduce a specific regeneration mixture.

According to the invention, the antiparticulate filter can be constructed of any porous material of known type, provided it resists temperatures of the order of 600-800° C.; however a filter of ceramic or metal material is preferably used.

In particular, a metal material possesses higher mechanical strength, a higher accumulation capacity, and high resistance to thermal stresses. It also enables thin-walled filters to be constructed, which ensure a high regeneration rate and also heat very uniformly because of the high conductivity of the material.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the invention will be apparent on reading the ensuing description provided by way of non-limiting example, with the aid of the figures of the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a system according to the invention for purifying the exhaust gases emitted by an automotive diesel engine;

FIG. 2 is a schematic illustration of an alternative embodiment of the system of FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

The system 1 comprises a plurality of exhaust collector tubes 10 each connected to a respective cylinder of a diesel engine 2, to convey the combustion-produced exhaust gases to a common meeting point 11, from which an exhaust pipe 12 extends to lead the exhaust gas flow to an exit 13 to atmosphere.

The architecture of the system 1 is purely indicative and can also be very different depending on the type of engine 2 and/or on the type of vehicle on which said system 1 is installed, without leaving the scope of the present invention.

A usual catalytic silencer 3 is located along the exhaust pipe 12, to be traversed by the flow of exhaust gases from the diesel engine 2, and contains suitable catalysts to activate nitrogen oxide (NOx) reduction and unburnt hydrocarbon (HC) oxidation.

According to the invention, upstream of said catalytic silencer 3, with respect to the flow direction, an antiparticulate filter 4 is positioned contained in a suitable casing, and able to retain the particulate carried in suspension by the exhaust gases, to prevent their release to atmosphere.

Preferably said antiparticulate filter 4 is constructed of a metal material, for example sintered metal. However it may also be constructed of any other porous material suitable for the purpose, for example ceramic material.

During operation, the antiparticulate filter 4 is traversed by the hot exhaust gases originating directly from the engine 2, before these cool substantially by flowing through the entire exhaust pipe 12.

In this manner it undergoes considerable heating, enabling substantially each region of the relative filtering walls to reach the minimum temperature for autocombustion of the regeneration mixture to take place during the regeneration step.

In fact, the extent of this heating varies according to the distance between the antiparticulate filter 4 and the engine 2, i.e. the overall length of the conduits which connect them together, as this determines the temperature of the exhaust gases passing through the antiparticulate filter 4.

Preferably, the antiparticulate filter 4 is located in proximity to the engine 2, such as to be heated to an average operating temperature at least exceeding 600° C.

By virtue of this arrangement, the conditions required for autocombustion of the normal exhaust gases emitted by the engine 2 can be recreated within the antiparticulate filter 4, to enable continuous regeneration in each operating stage, without the need to introduce specific regeneration mixtures. Some possible arrangements for achieving said temperature levels are considered hereinafter.

In the embodiment shown in FIG. 1, the antiparticulate filter 4 is located along the exhaust pipe 12, immediately downstream of the point 11 at which the exhaust collector tubes 10 meet, preferably upstream of any turbocompressor turbine if present.

In the embodiment shown in FIG. 2, the antiparticulate filter 4 is divided into a plurality of smaller filters 40, of a number equal to the number of cylinders of the engine 2, and each located along a respective exhaust collector tube 10 upstream of the meeting point 11. 

1. A system for purifying the exhaust gases emitted by an internal combustion engine (2), comprising a series-connected catalytic silencer (3) and antiparticulate filter (4) which are traversed in succession by an exhaust gas flow leaving the engine (2), characterised in that, with respect to the direction of said flow, the antiparticulate filter (4) is located upstream of the catalytic silencer (3) such as to be traversed by the hot exhaust gases from the engine (2) before they cool substantially.
 2. A system as claimed in claim 1, characterised in that said antiparticulate filter (4) is positioned in proximity to the engine, such as to be heated by the exhaust gases to an average operating temperature enabling continuous combustion of the particulate.
 3. A system as claimed in claim 1, characterised in that said antiparticulate filter (4) is positioned in proximity to the engine, such as to be heated by the exhaust gases to an average operating temperature exceeding 600° C.
 4. A system as claimed in claim 1, characterised by comprising an assembly of exhaust collector tubes (10) individually conveying the exhaust gases leaving a respective cylinder of the engine (2) to a common meeting point (11), the antiparticulate filter (4) being located immediately downstream of said common meeting point (11).
 5. A system as claimed in claim 1, characterised by comprising an assembly of exhaust collector tubes (10) individually conveying the exhaust gases leaving a respective cylinder of the engine (2) to a common meeting point (11), the antiparticulate filter (4) comprising an assembly of separate filters (40) individually located along a respective exhaust collector tube (10).
 6. A system as claimed in claim 1, characterised in that said antiparticulate filter (4) is of ceramic material.
 7. A system as claimed in claim 1, characterised in that said antiparticulate filter (4) is of metal material. 